It is beneficial to locate and track objects moving on ground and in air and/or space, and radar systems are commonly used for such tracking. For example, radar systems can be used to track commercial and military aircrafts, un-manned airborne vehicles such as drones, ballistic and cruise projectiles, speeding cars, etc. In many radar-based tracking systems, an electromagnetic signal or a pulse is transmitted in the direction of the suspected location of the moving object to be located and/or tracked. In some cases, e.g., when there is no suspected location, a spatial region may be swept with transmitted signals. Should a transmitted signal hit a moving object, that object would typically reflect that signal, and the reflected signal(s) can be detected by one or more receivers. The frequency of the reflected signal may be different from that of the transmitted signal due to the Doppler effect, where the change in the frequency is related to the speed of the moving object. The received signal(s) can be used to determine the location and/or speed of the moving object.
In some cases, the detection and/or tracking of an object can be defeated using a technique known as radar jamming. For example, the driver of a speeding car may use a radar jammer so that the speed of the car may not be determined by law-enforcement personnel. Jamming is often employed by an aircraft on a covert reconnaissance or attack mission or by a drone, so that these moving objects may not be located and/or tracked by an adversary. A digital radio frequency (RF) memory (DRFM) jammer is a commonly used jammer that includes a receiver, a DRFM, a transmitter, and a retro-directive antenna.
When a DRFM jammer is deployed on a moving object such as an aircraft, it can receive pulses from a radar and return them with modified apparent ranges and Doppler shifts, in order to inundate the radar with a large number of apparent, fake targets together with the actual reflection signal. This can make the detection and tracking of the aircraft more difficult. FIG. 1, obtained from http://www.mykonsult.com/ shows a simulated Plan Position Indicator (PPI) display for a relatively unsophisticated range-Doppler radar that has been inundated with several replica signals that are generated electronically by a DRFM jammer, along with the genuine reflection signal. While the genuine reflection signal appears as a target 102 in the PPI display, several other targets 104, each one corresponding to a replicated signal, also appear in the PPI display. Usually, the receiver and/or the radar operator cannot distinguish the actual target 102 from the fake targets 104. In some cases, because the strength of the replicated signals is stronger than that of the reflection signals, the genuine target may not even be recognized by the receiver and/or may not be displayed in the PPI display. Such a jammer generally represents an asymmetric threat because it is inexpensive and readily available even to an unsophisticated adversary.